A cobalt-chromium-molybdenum (Co—Cr—Mo) metallic alloy specified by ASTM F75 is commonly used for surgical implants such as for prosthetic knees, hips, shoulders, elbows, wrists, ankles, fingers, toes and spinal elements because of the alloy's strength, corrosion resistance, and biocompatibility. This Co—Cr—Mo alloy has greater wear resistance than stainless steels and titanium alloys. The nominal composition of the F75 alloy is 27.00 to 30.00% Cr, 5.00 to 7.00% Mo, 0.35% C maximum, 1.0% Si maximum, 0.50% Ni maximum, with balance of Co and other inevitable trace elements and impurities. All percentages herein are by weight, unless indicated otherwise.
The most commonly used materials in the friction pair of endoprostheses include metal-on-UHMWPE (ultra-high molecular weight polyethylene) (about 85%), ceramics-on-UHMWPE (7.2%), metal-on-metal (5.3%), and ceramics-on ceramics (2.5%). Many factors influence wear rates of materials pairs. These factors include the types of materials, contact stresses, surface hardness, surface roughness, type of articulation due to motion, number of cycles, solution particle count and distribution, oxidation of materials, and surface abrasions of both metal and polyethylene particulates.
As the articulating surfaces of orthopedic implants wear and corrode, products including plastic wear debris, metallic wear particles, and metallic ions will be released into the body, transported to and absorbed by bone, blood, the lymphatic tissue, and other organ systems. The polyethylene wear particles have been shown to produce long term bone loss and loosening of the implant. And, even very low concentrations of metallic wear particles and metallic ions are suspect in causing adverse toxic, inflammatory, and immunologic tissue reactions.
Although the ASTM F75 Co—Cr—Mo alloy is relatively well tolerated in the body, biological complications could occur which sometimes are due to the insufficient wear resistance of the alloy and the presence of the nano-size metal particles from wear debris of the metal-on-metal or metal-on-UHMWPE articulating joints. While ASTM F75 alloy is biocompatible in bulk form, it causes severe inflammatory reactions when nano-size particles are absorbed by tissue cells, potentially causing metallosis and tissue death. The only solution to the metal-on-metal problem is to reduce amount of wear debris. Hence there is a need for a non-toxic, non-allergenic, and biocompatible Co—Cr—Mo alloy with improved wear resistance over ASTM F75.
Metal allergy is an adverse reaction to the metallic ions which are released from an alloy by the action of sweat and other body fluids. In dentistry, Co, Cr, and Ni have been associated with metal allergy, and the use of Ni is rapidly being abandoned. M. Niinomi: Function Mater., 2000, vol. 20, pp. 36-44, reports metal allergy rates for Hg, Ni, Co, Sn, Pd, Cr, Cu, Pt, Zn, Au, Cd, and Sb.
The effects of metal ions released from orthopedic implants on nearby bone cells remain largely unknown. The problem is primarily the death of muscle tissue from the effects of metal ions, leading to bone loss. David A. Puleo, Winston W. Huh, Acute toxicity of metal ions in cultures of osteogenic cells derived from bone marrow stromal cells, Journal of Applied Biomaterials, Volume 6, Issue 2, pages 109-116, Summer 1995, revealed that Cr6+ was grossly cytotoxic; Co2+, Mo6+, Fe3+, and Ni2+ were moderately cytotoxic; and Ti4+, Al3+, V5+, and Mn2+ were minimally toxic. Ion solutions representing Co—Cr—Mo and 316 L stainless steel were moderately toxic; solutions representing Ti-6Al-4V were toxic at only the highest concentrations used. These results show that metal ions associated with Co—Cr—Mo and 316 L stainless steel are toxic to osteogenic cells at concentrations approximating those measured in the fibrous membrane encapsulating orthopedic implants.
In another study (G. C. F. Clark and D. F. Williams, The effects of proteins on metallic corrosion, Journal of Biomedical Materials Research, Vol. 16, 125-134 (1982)), the corrosion of the pure metals Al, Co, Cu, Cr, Mo, Ni, and Ti and of a Co—Cr—Mo casting alloy in buffered saline with and without the presence of the proteins serum albumin and fibrinogen was investigated. The corrosion of Al and Ti was unaffected by the protein. The corrosion rates of Cr and Ni showed a slight increase, while Co and Cu dissolved to a very much greater extent in the presence of the protein. The Mo demonstrated resistance to corrosion by the protein.
Pypen et al., Comparison of the cytotoxicity of molybdenum as powder and as alloying element in a niobium-molybdenum alloy, Journal of Materials Science: Materials in Medicine 9 (1998) 761-765), reported the cytotoxicity of the close-packed Nb metal and the Nb—Mo alloys based on a 72-hour direct contact test. Compared to a negative control (UHMWPE), Mo was moderately toxic; and Nb and Nb—Mo alloys were non-toxic.
Kennedy et al. U.S. Pat. No. 7,520,947 discloses work-hardening the low-carbon F75 type medical implant alloy by cold working such as by drawing followed by aging, or otherwise to impart wear resistance. For wrought shapes, a disadvantage of this is that substantial machining of the work-hardened wrought shape is required to yield the final product shape; which is especially difficult and expensive in view of the work hardening. For components produced by powder metallurgy or casting, the intricacy of the shape substantially complicates and in many instances renders impractical most work hardening.